We propose to study patients with antibiotic-induced and other gastrointestinal side effects of antibiotics to define any microbiological, immunological or toxic mechanisms responsible for these processes. Special attention will be given to the role of Clostridium difficile and its toxins. We will attempt to prevent or treat these conditions with various antimicrobial drugs or other substances found promising on the basis of these studies. Emphasis will be given to studies with oral vancomycin. We will study antibiotic-induced colitis in hamsters and other laboratory animals, examining in these models the same hypotheses as are being tested in humans, and with similar goals. We will attempt to extend these studies to other inflammatory bowel diseases of humans, such as ulcerative colitis and regional ileitis, and to determine the role C. difficile plays in the gastrointestinal side effects of cancer chemotherapy.